Neuromuscular electrical stimulation (NMES) (also referred to as powered muscle stimulation, functional muscle stimulation, electrical muscle stimulation, electrical stimulation, and other terms) is an established technology with many therapeutic uses, including pain relief, prevention or retardation of disuse atrophy, and improvement of local blood circulation. NMES is typically delivered as an intermittent and repeating series of short electrical pulses. In many implementations, these pulses are delivered transcutaneously by surface electrodes that are attached to a person's skin. Electrodes may be held to the skin through the use of straps, adhesives, or other mechanisms, and often contain a coupling layer composed of hydrogel that is capable of enhancing the efficiency of energy transfer from the electrode to the skin and underlying tissues.
A group of persons who could potentially show large benefit from NMES therapy are those who are being monitored medically for other conditions or as a standard part of their medical care. For example, many patients in the hospital are subjected to long periods of bed rest and develop atrophy that NMES could prevent. During hospitalization these patients are often connected to cardiac and other electrical monitors (ex. ECG) that measure/track certain aspects of the patient's health (for example, assessing potential arrhythmia and calculating heart rate). Similar statements may be made about subjects who might use NMES in other clinical settings or at home—electrical monitoring is often an important part of a patient's care that may drive either diagnostics or an intervention. Monitoring equipment may be external and temporary (ex. Holter monitor) or may be part of an implanted device that is permanent or semi-permanent. Monitoring and sensing capabilities may be stand-alone or integrated into another piece of medical equipment or device.
As NMES delivers electrical impulses to the body during its therapeutic application, there is the potential for interference with electrical monitoring equipment. For example, electrical signals delivered to the body as part of NMES treatment may be detected by sensors associated with other equipment, even in areas of the body remote to the site of NMES application. These NMES signals may alter or combine with (for example, through constructive or destructive interference) the physiological signals the sensors are intended to measure. If of sufficient amplitude, this interference may mask or alter signals detected by sensors in such a way that these signals are no longer reflective of the physiological event intended to be monitored. Accordingly, dangerous conditions may arise where a critical clinical event is not detected (for example, a cardiac arrhythmia goes undetected) or a device that implements sensing/monitoring behaves in an undesired fashion (for example, an implanted defibrillator shocks the patient during normal cardiac function because sensor interference is interpreted as a critical arrhythmia).
It is important to distinguish the situation currently described from the case of external electrical noise or other forms of external noise interfering with sensors. Sources of potential monitor interference arising from outside of the body are well-understood, and appropriate mechanisms are well-known in the art to prevent or limit substantial deleterious effects associated with these sources of noise. The presently-described situation, where sensor/monitor interference arises due to an electrical signal injected into or otherwise applied to the body, is much more challenging to handle and has limited available solutions.
The prior art illustrates some attempts to solve the interference problem described herein, but solutions described have inherent practical limitations. Solutions described in the prior art often use hardware or software-based signal filters that are applied to noisy data collected by sensors. Depending on the characteristics of both the desired and the interfering signals, these filters may be successful at removing the interference signal or minimizing its impact, allowing monitors and devices to function normally. Other solutions known in the art involve the use of additional sensors that are used in conjunction with the primary sensors associated with the monitor or device. These additional sensors may detect the interference signal, or a different combination of the desired and interference signals, and can be combined with data from the primary sensors in order to eliminate or minimize the impact of the interference signal on monitoring or sensing. For example, some systems described in the art use secondary sensors to measure the interference signal applied to the body, then subtract secondary sensor data from primary sensor data (which measures a combination of the desired physiological signal and the interference signal applied to the body) to minimize residual interference. Similar systems combine signals, sometimes from many additional sensors, in different ways (with or without the use of signal filters) to achieve similar goals.
The prior art systems noted above have practical limitations related to their implementation in the real-world. For example, to use filtering techniques to limit the impact of interference signals applied to the body, one would need access to sensor data following its detection but before it is interpreted, displayed, or otherwise used by algorithms/components later in a monitoring or device system's process. As a result, filters can be employed by the original manufacturers of monitoring equipment, but third-parties trying to prevent interference with existing equipment/monitors/sensors are prevented from implementing new filters as they generally do not have the proper access to make hardware or software modifications to existing equipment. Similar limitations are associated with a multiple-sensor approach; even in cases where the use of multiple sensors could help eliminate interference with measurements of physiological signals, these sensors cannot generally be added post-market to existing monitors or devices that measure and interpret data.
As one specific example, take the case of a patient in a hospital having his cardiac signals monitored with standard ECG equipment. ECG signals are measured by sensors (ECG electrodes) applied to the body and relayed to a processor/display unit via conventional leads that are well-known. If NMES is applied to the patient, signals detected by ECG electrodes may be a blend of the cardiac signals desired to be measured and an interference electrical signal produced as a byproduct of NMES treatment. Even if the NMES interference signal could be isolated and measured exactly with secondary sensors, there is no way to adjust the ECG electrode data with information from the secondary sensors without major modifications to the ECG monitoring system. In other words, the ECG sensor data is ported directly to the processer/display unit, and there is no practical way to intercept this data and adjust it using information from a secondary sensor before it is interpreted and displayed. Similar limitations are associated with the use of the filter approach. Thus, there is no way to prevent this type of signal interference using these methods without working directly with the ECG monitor manufacturer to implement them. As there are a vast possibility of devices and monitors that could suffer from interference from NMES devices and other devices that supply electrical signals to the body, collaborating with each manufacturer to implement to the techniques described in the prior art is impractical and thus these solutions aren't feasible for widespread use.
Novel solutions are needed to allow NMES and other devices to be used safely in the presence of monitoring and sensing equipment. These new approaches must solve the practical problems described above, and allow for interference reduction to be implemented in such a way that no modifications to monitoring devices are needed in order to reduce the interference produced by the therapy devices and subsequently detected by the sensors on the monitoring devices. Disclosed within are devices, systems, and methods for achieving these goals.